${}_{}{}^{115}{}_{}{}^{}\mathrm{In}$ nuclear-magnetic-resonance Knight shift studies in ${\mathrm{Yb}}_{\mathrm{x}}$${\mathrm{In}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Cu}}_2$ microscopically confirm the existence of an anomalous valence transition of Yb ions for some values of x. The rate of variation, as well as the overall variation of the Knight shifts is the largest ever observed in any Yb system. The Knight shifts do not track the susceptibility at low temperatures, the possible implications of which are discussed. Specific-heat results prove the intermediate-valence character of Yb ions in the ground state of these compounds.

• Received 3 September 1986

DOI:https://doi.org/10.1103/PhysRevB.35.2035